FIG. 10 illustrates a conventional high-frequency signal receiver. A high-frequency signal is received at an input port 601 and transferred to an automatic gain control (AGC) circuit 602, a variable gain amplifier for controlling the level, the amplitude, of the signal. A mixer 605 receives a signal output from the AGC circuit 602 and a signal output from a local oscillator 604. A signal output from the mixer 605 is transferred to a filter 607. An AGC controller 606 controls a gain of the AGC circuit 602 through a gain control port 603 according to a signal output from the mixer 605. A signal output from the filter 607 is transferred to an AGC circuit 608. A mixer 610 receives a signal output from the AGC circuit 608 and a signal output from a local oscillator 609. A signal output from the mixer 610 is transferred to a filter 612. An AGC controller 611 controls a gain of the AGC circuit 608 according to a signal output from the filter 612. A signal output from the filter 612 is transferred to an AGC circuit 613. A signal output from the AGC circuit 613 is received by an AD converter 614, and a signal output from the AD converter 614 is transferred to a digital filter 615. A signal output from the digital filter 615 is transferred to a demodulator 617. A signal output from the demodulator 617 is output through an output port 618. An AGC controller 616 controls a gain of the AGC circuit 613 according to a signal output from the digital filter 615.
An operation of the conventional high-frequency receiver having the above arrangement will be explained. It is assumed that a first intermediate frequency output from the mixer 605 is higher, than the frequency of the input signal, and a second intermediate frequency output from the mixer 610 is lower than the frequency of the input signal.
A high frequency signal, for example, digitally modulated ranging from 90 MHz to 770 MHz is input to the input port 601. The input signal is then amplified by the AGC circuit 602 and transferred to the mixer 605 for mixing the signal and a signal output from the local oscillator 604 to produce a signal at a first intermediate frequency, e.g. 1200 MHz. The signal at the first intermediate frequency is then received by the AGC controller 606. A voltage output from the AGC controller 606 is fed to the AGC circuit 602 for controlling the gain of the AGC circuit 602 for maintaining the level of the signal output from the mixer 605 in constant.
The high frequency signal output from the mixer 605 has an undesired signal suppressed by the filter 607, is amplified by the AGC circuit 608, and is transferred to the mixer 610 for mixing the signal and a signal output from the local oscillator 609 to produce a signal at the second intermediate frequency, e.g. 4 MHz.
The signal of 4 MHz output from the mixer 610 has an undesired signal suppressed by the filter 612, and is received by the AGC controller 611. A voltage output from the AGC controller 611 is fed to the AGC circuit 608 for controlling the gain of the AGC circuit 608 for maintaining the level of the signal output from the mixer 610 in constant.
The signal at the second intermediate frequency output from the filter 612 is amplified by the AGC circuit 613 and is converted into a digital signal by an analog/digital (A/D) converter 614. The digital signal has an undesired signal suppressed by the digital filter 615 and is demodulated by the demodulator 617, thus being output from the output port 618.
The signal output from the digital filter 615 is received by the AGC controller 616. A voltage output from the AGC controller 616 is fed to the AGC circuit 613 for controlling the gain of the AGC circuit 613 for maintaining the level of the signal to be received by the demodulator 617 in constant.
Specifically, the gains of the AGC circuits 602, 608; and 613 are determined according to the level of the input signal received at the input port 601. This arrangement provides the signal output finally at a good carrier-to-noise (C/N) ratio and a reduced distortion which may be caused by interference signals adjacent in frequency to the output signal. The noise factor (F) of the high-frequency signal receiver is expressed as:
                    F        =                              F            1                    +                                                    F                2                            -              1                                      G              1                                +                                                    F                3                            -              1                                                      G                1                            ·                              G                2                                              +          …                                    (                  Equation          ⁢                                          ⁢          1                )            where
F. A noise factor of the AGC circuit 602.
“G1: The gain of the AGC circuit 602, F2: A noise factor of the mixer 605, G2: A total gain of the mixer 605 and the filter 607, and F3: A noise factor of succeeding circuits including the AGC circuit 608”
The C/N ratio of the high-frequency signal receiver is expressed as
                              C          N                =                  Psi                      kTB            ⁡                          (                              F                -                1                            )                                                          (                  Equation          ⁢                                          ⁢          2                )            where
Psi: The level of the desired signal (W),
k: The Boltzmann constant, 1.38×10−23 (J/K),
T: An ambient temperature (K), and
B: A frequency range of the desired signal (Hz).
Equation 2 indicates that the C/N ratio of the high-frequency signal receiver is determined by the level (Psi) of the desired signal and the noise factor (F).
For example, the AGC controllers 606, 611, and 616 are preset to control the gain of the AGC circuit 602 when the level of the high frequency signal received at the input port 601 is larger than −70 dBm, and to control the gain of the AGC circuit 608 when the level is not larger than −70 dBm.
The C/N ratio of the high-frequency signal receiver will be explained when the input signal contains only the desired signal or contains the signal and a small adjacent signal adjacent to the desired signal. Such interfering signals adjacent to the desired signal are classified into the adjacent signal and an adjacent-adjacent signal. The following description is based on the adjacent signal.
When the level of the adjacent signal is smaller than the level of the desired signal, the gain is controlled according substantially only to the desired signal.
FIG. 3 illustrates the noise factor F of the high-frequency signal receiver against the level of the input signal. When the level the desired signal of the high-frequency signal stays in a range 301, not larger than −70 dBm, the gain G1 of the AGC circuit 602 is at its maximum as calculated by equation 1. Accordingly, the noise factor F is determined by the noise factor F1 of the AGC circuit 602 and remains low as a curve 305. Then, when the level of the desired signal shifts into a range 302, larger than −70 dBm, the gain G1 of the AGC circuit 602 is controlled to shift lower. The noise factor F2 of the mixer 605 and the noise factor F3 of the succeeding circuits including the filter 607 do not become negligible accordingly, and therefore, the noise factor F of the high-frequency signal receiver significantly increases as a curve 303.
FIG. 4 illustrates the C/N ratio of the high-frequency signal receiver in relation to the level of the desired signal. When the desired signal is in a range 401, not larger than −70 dBm, the noise factor 303 remaining in constant throughout the range 301, as shown in FIG. 3 while the level (Psi) of the desired signal increases. Accordingly, the C/N ratio defined by equation 2 increases according to the level (Psi) of the desired signal as a curve 405. When the desired signal stays in a range 402, larger than −70 dBm, the noise factor F of the high-frequency signal receiver increases substantially in proportion to the signal level (Psi) of the desired signal, as shown throughout the range 302 in FIG. 3, and the C/N ratio defined by equation 2 remains in constant as shown by a line 403.
The C/N ratio of the high-frequency signal receiver will be explained when the level of the adjacent signal is larger than that of the desired signal.
FIG. 5 illustrates the C/N ratio of the high-frequency signal in relation to the level of the adjacent signal while the level (Psi) of the desired signal remains in constant at −70 dBm. When the level of the adjacent signal is in a range 501, smaller than −70 dBm, the noise factor F of the high-frequency signal receiver is small, as shown by the line 304 in FIG. 3, since the gain is controlled by the level of −70 dBm of the desired signal. Accordingly, the C/N ratio defined by equation 2 stays in constant, as shown by a line 504 in FIG. 5.
In the case that when the desired signal has the level of −70 dBm, when the adjacent signal shifts into a range 502, larger than −70 dBm, as shown in FIG. 5, the noise factor F of the high-frequency signal receiver increases substantially in proportion to the signal level (Psi) of the input signal, as shown in FIG. 3, and the level (Psi) of the desired signal remaining in constant at −70 dBm. This allows the C/N ratio defined by equation 2 to decrease and decline according to an increase of the level of the adjacent signal. Accordingly, the C/N ratio gradually declines, as shown by the line 503, in reverse proportion to the level of the adjacent signal.
The adjacent signal deteriorates the C/N ratio according to the level of the signal, and generates a three-dimensional inter-modulation distortion (referred to as IM3 hereinafter). The deteriorating for the C/N ratio affects the receiver more than the distortion.
Under the condition that the level of the adjacent signal is larger than that of the desired signal of the input signal, the gain of the AGC circuit 602 can be controlled to be lower according an increase of the level of the adjacent signal. As the result, the C/N ratio of the high-frequency signal receiver declines, thus increasing a bit error rate (BER) and interrupting the reception of the desired signal.
A conventional mixer for suppressing an image interference disclosed in Japanese Patent Laid-Open Publication No. 8-288879 prevents the receiver from receiving the desired signal of the input signal when an interference component adjacent to the desired signal has a level, e.g. 20 dBm, larger than the level (Psi) of the desired signal.
Alternatively, a microcomputer may measure the C/N ratio or the bit error rate at the demodulator 617 to control the gain of the AGC circuit 602 with the measured ratio or rate for reducing an influence of the adjacent signal. However, This controlling operation takes a considerably-long time, thus being unfavorable particularly for receiving a signal while a condition of the received signal changes in time, for example, during moving.